From U.S. Pat. No. 3,080,134 it is known to provide a machine part with a hard ceramic coating having predetermined surface qualities, with regard to the micro-structure of the surface (rounded rather than sharp-edged elevations) as well as the degree of the resulting roughness. From U.S. Pat. No. 3,902,234, it is known to provide a drawing roll used during spinning of synthetic yarns with a ceramic coating according to U.S. Pat. No. 3,080,134 and thereafter to deposit metallic platinum or metallic palladium between the elevations in the ceramic coating surface. These metals have the effect of catalysts to enhance the decomposition of deposits on the surface of the draw roll through the influence of oxygen from the air. The decomposition is achieved through a slow, little by little, "burning" of the deposits at temperatures below the melting temperature of the synthetic material.
The concept of such a catalyst coating is excellent. The realization of such a coating according to U.S. Pat. No. 3,902,234, however, shows certain drawbacks. This patent teaches that the catalytic metal should, after a chemical reaction in a solution, deposit itself in the form of particles between the elevations in the surface of the ceramic coating. The process has to be carried out in several steps, since the reactive components have to be applied on the object one at a time, after which the object must be heated, in order to accelerate the reaction. Since the deposition can not be carried out selectively, a layer of the catalyst is formed between the elevations as well as on them. Generally, only a very loose bond develops between the metal and the ceramic surface, so that the outer part of the metal coating can easily be brushed off, which leaves the desired catalytic deposits between the elevations, however with the loss of the removed catalyst material. Even if the latter material is used again, this further complicates the process. Additionally, the particles have to be very fine in order to lay between the elevations. Such fine particles can enter the lungs, which would be particularly dangerous if the particles are needle-shaped. Furthermore, platinum requires special treatment anyhow, because for platinum-bonds extremely low MAK values apply (MAK=Maximum-Arbeitsplatz-Konzentration/maximum workplace concentration).
It is known from U.S. Pat. No. 3,266,477 to provide cooking utensils with catalyst coatings in order to obtain a "self-cleaning" effect. For this one can expect relatively high operating temperatures (200 to 260.degree. C.) and the catalyst layer is obviously designed as a non-interrupted coating of the protected surface, even though the possibility of an interruption is mentioned at column 5, line 32 of the patent. Different types of catalysts are considered, some of which can be applied by means of plasma spraying (column 2, line 30). Precious metals, even though they are mentioned as catalysts in U.S. Pat. No. 3,266,477, are apparently not used for spraying.
It is also known that it is for instance possible to spray platinum, as is mentioned in a paper ("Plasma Spraying Technique--Basics and Applications") by Plasma Technik AG, 5607 Hagglingen, Switzerland. The production of platinum catalysts by means of spraying however, is rather unusual. It is, for instance, not mentioned in the context of Rompp's Chemie Lexikon, (Encyclopaedia of Chemistry) published by Keller, Stuttgart, Germany (edition 8, 1987, page 3256). Known (in U.S. Pat. No. 3,136,658 and U.S. Pat. No. 3,125,539) is the application of platinum protective layers by means of wire-spraying, wherein platinum is furnished in the form of wires to metal spray guns which fuse, atomize and spray the platinum. In U.S. Pat. No. 3,125,539 an application of platinum in the form of powder is also mentioned (column 3, line 16). The production of protective layers using materials from the platinum family is, however, not the object of this invention.
The following possibilities are also known from the known art:
______________________________________ EP-A-423063- A protective layer is formed from a metallic matrix, wherein carbide and oxide ceramic- particles are embedded to serve as resistive substances. During the coating process oxide particles can be produced by way of a so called method of high-speed-flame-spraying. EP-A-54165- Printing drum with complex layer structure, with an (inner) layer of TiO.sub.2 with 3% platinum applied with the method of plasma spraying. Platinum is used as an additive to obtain electric conductivity. GB-2130250- Production of a bearing alloy (e.g. from Al/Pb), by using a method of plasma spraying to build up layers and whereby the portion of one component increases steadily. EP-A-223104- Application of a protective layer consisting of aluminum silicate. For this the method of plasma spraying can be applied. DE-372 1008- In a bearing alloy similar to the alloy according to GB 2130250, parts of a resistive substance are added to the plasma stream. US-3279939- A layer having a high wear resistance at high performance temperature (e.g. above 1000.degree. F.) can be formed by a method of spraying. EP-A-592310- A layer with a low friction coefficient can be formed by a method of thermal spraying. EP-A-401611- A layer for protecting against corrosion and wear consists of high temperature compound material, and can be formed through building- up-welding or the method of plasma spraying. Platinum is mentioned as an additive substance for improvement of the corrosion stability and for stabilizing the carbides. US-3020182- Formation of ceramic/metal bond through thermal spraying. ______________________________________
The above listed examples show that the application of the method of thermal spraying has been known for a long time. Nevertheless, a catalytic effect is not mentioned.